I don't think it's NIH so much as the glibc ABI is a broad surface of implementation specific choices outside the C standard, and a moving target. It's an emergent property of many small implementation choices that another libc project would find too difficult to replicate.

I had a recent experience fighting this sort of thing when I wanted to cross-compile from a distro using glibc to a distro using musl.

Glibc is also kind of bloated and it's reasonable that android or a similar embedded project would not want to pull the whole thing in. Especially when you consider the original goal of bionic was just to bootstrap Java code, which was meant to be the primary platform interface rather than C, they would want a very minimalist libc for that purpose.